The parity principle is founded on the norm of equal justice: Green & Quinn v The Queen [2011] HCA 49 [28]. Its terms, in the ordinary case, are settled. It was expressed in this way by Gibbs CJ (Wilson J agreeing) in Lowe v The Queen [1984] HCA 46; (1984) 154 CLR 606:
It is obviously desirable that persons who have been parties to the commission of the same offence should, if other things are equal, receive the same sentence, but other things are not always equal, and such matters as the age, background, previous criminal history and general character of the offender, and the part which he or she played in the commission of the offence, have to be taken into account (609).
See also I (a child) v The State of Western Australia [2006] WASCA 9 [65] - [71] (Steytler P).
The concept of equal justice does not equal mathematical precision. What is required before the court can intervene is that any disparity (or lack of disparity) gives rise to an objectively justifiable sense of grievance or an appearance that justice has not been done. The fact that an appellant feels a sense of grievance is not determinative: Postiglione v The Queen [1997] HCA 26; (1997) 189 CLR 295, 301 (Dawson & Gaudron JJ), 338 (Kirby J).
Ultimately, what is required is that there is a proper relationship between the sentences imposed on co-offenders. This requires a comparison of the sentence imposed on each offender, and an evaluation of their involvement in the commission of the offence and their antecedents: D A Thomas, Principles of Sentencing (2nd ed) 64 - 65; and Jardim v The State of Western Australia [12] - [13] (McLure P, Pullin JA agreeing), [22] (Hall J) [41] - [43].